This invention relates to discrete multi-tone (hereinafter xe2x80x9cDMTxe2x80x9d) communication systems. Particularly, the invention concerns improved apparatus and methods for controlling data transmissions on discrete multi-tone communication systems.
Discrete multi-tone communication systems transmit information encoded in multiple tones, where each tone corresponds to a modulated carrier signal having a different frequency. For instance, a full-rate DMT system conforming to the ITU G.922.1 (also known as G.dmt) standard use 255 different tones corresponding to a bandwidth of 1.104 MHz. Other DMT systems, referred to as half-rate DMT systems, conform to the ITU G.992.2 (also known as G.lite) standard and use 127 different tones corresponding to a bandwidth of 0.552 MHz. A DMT system can be viewed as a group of multiple voice band modems (256 modems in the case of full-rate DMT) with their carrier frequencies ranging from a few kHz to near 1 MHz.
In typical distributed DMT systems based on either the G.dmt or the G.lite standard, tones #1-31 are reserved for upstream transmissions from the client (e.g. consumer personal equipment) to the Central Office (hereinafter xe2x80x9cCOxe2x80x9d). The remaining tones (i.e. tones #32-255 for G.dmt) are reserved for downstream transmissions from the CO to the client. If echo cancellation is supported at both the Central Office and the client, then the DMT system can operate in an overlapped spectrum mode where tones #1-31 are used for both upstream and downstream transmissions.
At the beginning of each connection, certain initialization operations (e.g. transceiver training, channel analysis, message exchange, and bit loading) are performed so that each connection between Client and Central Office can identify which of the tones are available for transmission during the datamode transmissions (i.e. non-initialization transmissions). After initialization, the DMT system can begin transmitting data.
Known DMT systems can also include a pilot tone to aid in timing recovery during the initialization phase of the DMT system. A pilot tone can be implemented by using a subcarrier dedicated only to timing recovery. In particular, the DMT system can provide for transmission of a pilot tone at one DMT transceiver, called the clock source, and the DMT system can provide for receipt of the pilot tone at another end of the DMT transceiver, called the clock recipient. By comparing the pilot tone with the clock at the clock recipient the DMT system can be synchronized.
DMT systems can be implemented by connecting a DMT transceiver with a Central Office through a twisted-pair telephone loop typically used to connect Plain Old Telephone Systems (hereinafter xe2x80x9cPOTSxe2x80x9d). DMT systems can be designed to coexist with a POTS service on the same twisted-pair cable. A Frequency Division Multiplex scheme is necessary to separate the voice channel for POTS from the spectrum occupied by a DMT system, such as in a Digital Subscriber Line (hereinafter xe2x80x9cDSLxe2x80x9d). Thus, communication systems are known that connect a POTS service with a DMT system on the same twisted-pair cable, however, systems connecting two DMT transceivers to the Central Office on the same twisted-pair cable are not known.
Connecting two or more DMT transceivers to the Central Office on the same twisted-pair cable may prove advantageous in locations already wired for POTS, such as existing homes. Currently, if two DMT transceivers were connected to the Central Office on the same twisted-pair cable, the interference between the two DMT signals would disrupt communication with the Central Office.
Accordingly, there exists a need for a DMT communication system capable of connecting two DMT transceivers to the Central Office on the same cable.
A selected remote end terminal chosen from a plurality of remote end terminals in a DMT system may be connected to a Central Office, according to the invention, by sensing data transmission activity and by transmitting data from the selected remote end terminal when no data transmission activity is sensed. The plurality of remote end terminals may be connected together at a node, and the node can be connected to the Central Office through a cable, such as a telephone wire. The selected remote end terminal senses data transmission activity between the Central office and another of the plurality of remote end terminals connected to the node. The selected remote end terminal then transmits data when no data transmission activity is sensed between the Central Office and another of the plurality of remote end terminals.
In another aspect of the invention, a DMT system includes a selected remote end terminal connected to a Central Office, the selected remote end terminal including a discrete multi-tone transceiver and a controller for controlling the transmission of data from the discrete multi-tone transceiver. The discrete multi-tone transceiver receives data from the Central Office and the discrete multi-tone transceiver transmits data to the Central Office over cable, such as a telephone wire. The controller controls the transmission of data from the discrete multi-tone transceiver in response to data transmission activity between the Central Office and another remote end terminal.